The present invention relates generally to the field of coal gasification and, in particular, to a steam generator used to contain and cool the synthesis gas produced by coal gasification processes.
Integrated gasification combined cycle (IGCC) power plants firing solid fuels have traditionally been higher capital cost and have had lower operating availability and reliability than competing solid fuel technologies such as pulverized coal combustion Rankine cycles. Primary components to be improved upon to make IGCC more competitive include uncooled gasifiers and radiant and convective synthesis gas coolers. Radiant synthesis gas cooler designs have a practical limitation of overall outside diameter due to the economics of pressure vessel containment and shipping size limitations to most power plant sites. Within these limits to vessel diameter, there is a need to maximize the compactness of the radiant heat transfer steam generating surface used to cool the gas to minimize the overall height of the radiant synthesis gas cooler.
U.S. Pat. No. 4,768,470 to Ziegler utilizes coaxial flues constructed of steam generating wall surface to shorten overall cooler height. This design provides for separate flues with independent water circuits to provide for individual lifting, removal and inspection of the inner and outer flues. Another design approach developed by The Babcock & Wilcox Company ca. 1992 utilizes a single flue of steam generating wall surface with additional steam generating surfaces (“wing walls”) suspended inside the flue to maximize surface area and shorten cooler height. Other companies, such as GHH Mann employ similar designs.
Existing solutions still have not reduced the cost of this component to a competitive level. Single radiant cooler heights to cool synthesis gas for power plants using the largest commercial gas turbines can exceed 150 feet tall. Some plant designs have utilized two coolers, reducing overall height but further increasing costs. Additionally, redundant gasifiers, radiant coolers and convective coolers have been included in plant designs to improve plant operating availability, at a substantially higher cost.
Existing solutions for convective synthesis gas coolers require a separate component from the radiant cooler, with a cooled flue connecting the two components. Convective coolers designs include both water and steam tube designs (water or steam inside the tubes, gas outside) (Shell Oil Company) and fire tube designs (gas inside the tubes, water outside) (Steinmueller, others). Both of these designs require a pressure vessel enclosure and water/steam system, separate from the radiant cooler. Turbulence created in turns in the gas flue and at the inlet to the convective cooler have created a source of fuel ash fouling that can be difficult to manage.
Existing solutions for gasifiers include uncooled and cooled refractory enclosures. Uncooled enclosures (General Electric, Conoco, others) have experienced premature failures and frequent replacement. High availability with these designs typically requires a spare gasifier train, and/or firing the gas turbine on oil or gas at higher cost during repair time for the gasifier. Slow heat up and cool down times for thick refractory uncooled designs extend time during outages to repair or replace refractory. Existing cooled gasifier designs (Shell Oil Company, Future Energy) utilize separate water or steam generating circuits with a refractory coating to enclose and contain the gasifier gases. Some of these systems use low pressure, forced circulation cooling water systems that reject the heat outside of the power plant steam/water system, reducing efficiency. Prior art for containing hot solid fuel gases with molten slag in a combustion environment similar to this environment using steam generating surface integral with the downstream cooling circuitry includes Cyclone™ fired boilers (The Babcock & Wilcox Company).
It is thus clear that development of an economical, compact, reliable and robust synthesis gas cooler is critical to the future of IGCC systems at a commercial scale.